Existing technologies enable wireless detection of objects near a reader device. For example, the effective range of BLUETOOTH brand devices is about 10 meters, while some radio frequency identification (RFID) readers can detect RFID tags on objects as far as 100 meters away. Various polling schemes have been proposed for the BLUETOOTH brand devices to maintain contact, but the schemes require frequent polling of devices and thus carry a significant power cost. Similarly, some RFID readers frequently poll for the RFID tags to determine whether any of the expected RFID tags are not detected. In such systems, the RFID readers are typically mounted in a fixed array, and each of the RFID readers has a dedicated and continuous power source to satisfy the significant power requirements of the frequent polling.